Vehicle radiators have a pair of coolant tanks located to either side of a tube and fin core, each of which is substantially filled by liquid coolant. Most often, the tanks are oriented vertically, with the core tubes running horizontally between. It is occasionally necessary to drain the coolant, and the lower end of a tank is the most convenient location for a draincock assembly to do so. The usual draincock assembly includes a tubular valve body protruding generally horizontally from the tank with an open outer end. A drain tube protrudes from the valve body approximately perpendicularly thereto, opening through the inner surface of the valve body. A valve is inserted through the open outer end of the valve body, and is movably supported the valve body so as to translate back and forth, when twisted by an operator, between a closed position blocking the drain tube and an open position. In the closed position, a rubber O-ring carried on the valve is radially compressed into the inner surface of the valve body, blocking flow to the drain tube. When open, the O-ring is pulled back past the drain tube, and coolant can run through the valve body and into the drain tube and out.
To assure that coolant drains only through the drain tube and does not also leak out of the valve body, some means must also be provided to block the outer end of the valve body when the valve is open. In many cases, the valve is threaded into the valve body and the threads themselves block back flow sufficiently. Consequently, the inner surface of the valve body can be enlarged in diameter outboard of the drain tube opening, so that the O-ring is taken out of compression when the valve opens. However, a new valve design translates the valve body not with threads, but with a pin and cam slot. An allowed coassigned patent application Ser. No. 07/901,298 now U.S. Pat. No. 5,246,202 in the name of Henry Beamer covers this particular valve design. While this is a less expensive design to produce, and provides some other advantages over a threaded valve body, it does nothing inherently to prevent leakage out of the end of the valve body when the valve is open. Therefore, it is necessary that the O-ring be kept constantly in compression, both in the open and closed position. One consequence of this is that the compressed O-ring, as it is pulled past the edge of the aperture, can bulge out into the drain tube slightly and be damaged or cut.